1. Field of the Invention
This invention relates to the production of ceramic-grade uranium dioxide powder from UF.sub.6. More particularly, it pertains to the substantially complete conversion of uranium hexafluoride to uranium dioxide by a continuous procedure involving three fluidized beds interconnected in series. Both enriched uranium hexafluoride and natural uranium hexafluoride as well as depleted uranium can be processed.
2. Description of the Prior Art
Uranium dioxide is a desirable fuel in nuclear reactors because of its excellent physical and chemical properties, such as inertness to cooling media, irradiation stability, and retention of fission gases. Having these properties uranium dioxide is suitable for use in both bulk and dispersion forms. Moreover, it is relatively easy to prepare and to fabricate into desired compacts, as well as being amenable to chemical reprocessing after irradiation. With the growing use of nuclear fuel in the electrical power field, a reduced cost of preparation would provide added incentive for further employment of uranium dioxide as fuel material.
One of the expedient methods for producing uranium dioxide is by the conversion of uranium hexafluoride. One method is that disclosed in U.S. Pat. No. 2,906,598 which primarily involves aqueous precipitation, filtration, drying, calcination, and reduction steps. Unfortunately, aqueous procedures ordinarily do not permit control of conditions that provide a uranium dioxide powder having the desired combination of uniformity and good sinterability and at a low cost. For that reason, fluid-bed processes are being more widely considered for the conversion of uranium hexafluoride to uranium dioxide powder.
One disadvantage of fluid bed processes having only one stage, as in U.S. Pat. No. 3,160,471, or two stages, as in U.S. Pat. No. 3,547,598, for converting UF.sub.6 to UO.sub.2 has been the relatively high residual fluorine values. Fluorine in UO.sub.2 powder is undesirable because it sometimes adversely affects the sinterability of the powder. Further, although some fluorine is removed during the sintering operation, the fluorine has a corrosive effect on the sintering furnace parts and is highly undesirable in fuel elements because of its attack on the metal cladding. Unless the residual value of fluorine remaining in the sintered fuel pellet is extremely low, it reduces the life of the cladding in which the fuel pellet is encased. Moreover, such prior known gas phase processes have resulted in disappointingly low yields of enriched uranium recovery as UO.sub.2 which is costly and wasteful, which fact has indicated a need for a more efficient method for recovering all of the uranium.
U.S. Pat. No. 3,179,491 discloses a single step process in which uranium yields of 89 to 91% are indicated which means about a 10% loss of the costly uranium.
U.S. Pat. No. 3,235,327 discloses a two-stage process which is dependent upon the use of relatively high volumes of nitrogen gas as a critical factor in the success of the disclosed process.
U.S. Pat. No. 3,168,369 discloses gas phase reaction of UF.sub.6 and steam in a first reactor to produce very fine UO.sub.2 F.sub.2 powder. The resulting UO.sub.2 F.sub.2 powder must then be reacted in a calciner with hydrogen and steam to produce UO.sub.2, with nitrogen gas being introduced. The recovery of the fine UO.sub.2 F.sub.2 powder of 0.1 micron size and finer is very difficult and requires multiple recovery systems in series.